1. Field of the Invention
This invention relates to an efficient coding method/coding apparatus for digital audio data which are adapted for carrying out coding of input digital data by the so-called efficient coding to transmit, record, reproduce or decode coded data to provide a reproduced signal, and to a recording medium on which data formed by these coding method/apparatus is recorded.
2. Description of the Related Art
There are various efficient coding techniques for an audio signal or a speech signal, etc. For example, in ICASSP 1980 The critical band coder--digital encoding of the perceptual requirements of the auditory system M. A. Kransner MIT, a technique is described in which the hearing sense masking is utilized to provide necessary signal-to-noise ratios every respective bands to carry out fixed bit allocation. With this technique, however, in the case of measuring the characteristic by a sine wave input, because the bit allocation is fixed, the characteristic value is not so good.
To solve such problem, there is proposed an efficient coding apparatus in which all bit which can be used for bit allocation are used in such a manner that they are divided into bits of a fixed bit allocation pattern determined in advance every small blocks and bits of a variable bit allocation pattern for carrying out bit allocation dependent upon magnitudes of signals of respective blocks to allow the divisional ratio to be dependent upon a signal related to an input signal, thus to allow the divisional ratio with respect to the fixed bit allocation pattern to be greater according as the spectrum of the signal becomes smooth to more degree.
In accordance with this method, in the case where an energy concentrates on a specific spectrum component as in the case of a sine wave input, a larger number of bits are allocated to a block including that spectrum component, thereby making it possible to remarkably improve the entire signal-to-noise characteristic. Since the hearing sense of the human being is generally extremely sensitive to a signal having a steep spectrum component, improving the signal-to-noise characteristic by using such a method not only results in improvement in a numeric value in measurement, but also is effective for improvement in the sound quality from a viewpoint of the hearing sense.
However, if allocation of bits dependent upon an input signal is carried out solely on the basis of an improvement in the signal-to-noise characteristic, in the case of attempting to compress a signal including a large number of steep spectrum components like sound of a triangle, for example, at a low bit rate, sufficient bits cannot be allocated to blocks corresponding to respective spectrum components, failing to provide a sufficient sound quality from a viewpoint of the hearing sense.
In view of this, the applicant of this invention has already proposed in the U.S. patent application Ser. No. 08/011,376 (filed on Jan. 29, 1993; now U.S. Pat. No. 5,301,205) a technique in which all bits which can be used are used in such a manner that they are divided into bits of a fixed bit allocation pattern determined in advance with respect to any short time and bits of a variable bit allocation pattern for carrying out bit allocation dependent upon the magnitudes of signals of respective blocks. Namely, bit allocation is carried out such that weighting is conducted in accordance with corresponding bands of blocks with respect to not only the fixed bit allocation pattern but also variable bit allocation pattern dependent upon magnitudes of signals of respective blocks, thus to solve the above-described problem.
Here, weighting is effective such that a larger number of bits are allocated to blocks on a lower frequency band side by taking into consideration the fact that noise on a higher frequency band side is difficult to strike or catch the ear of the human being than noise on a lower frequency band side because of sensitivity of the ear of the human being, and is apt to be masked by signals on the lower frequency band side.
On the other hand, when a signal of a large amplitude (in the vicinity of 0 dB) is inputted to compressing devices using the previously described various techniques, an overflow takes place when that signal is converted to a signal of a finite word length at the output section of the expanding device by a change of the level resulting from the influence of an error in quantization, so a higher harmonic wave by clipping is produced. This higher harmonic wave has a frequency lower than that of a signal inputted to the compressing device by aliasing when converted to an analog signal. As a result, such higher harmonic wave is recognized as noise for the human being, resulting in an obstacle from a viewpoint of the hearing sense. For such problem, there is readily conceivable a technique to make such an adjustment to amplitude-suppress or attenuate in advance a level change resulting from an error in quantization, or the like in the compressing device to thereby prevent overflow in the expanding device.
However, in a compressing device utilizing the characteristic in the hearing sense of the human being as previously described, relatively small quantization noise on a lower frequency band side and relatively great quantization noise on a higher frequency band side result from the characteristic.
Accordingly, in the case where the above-mentioned technique of suppressing or attenuating amplitude is employed, when an adjustment to such an extent capable of preventing an overflow resulting from quantization noise on the higher frequency band side is carried out, a change in the level by correction becomes equal to a quantity which cannot be neglected on the lower frequency side, i.e., in a frequency band sensitive from a viewpoint of the hearing sense of the human being, resulting in the possibility that there occurs an obstruction from a viewpoint of the hearing sense.
On the contrary, if a correction is made to such an extent that a level change by correction on the low frequency band side is negligible, there occurs the possibility that an overflow by quantization error on the higher frequency band side cannot be prevented.
Further, since quantization error in the compressing device varies at any time adaptively with respect to change in the amplitude in point of time, frequency distribution and power or energy, etc. of an input signal, correction of a maximum quantization error which can be produced must be carried out in order to completely prevent an overflow in the expanding device. Since this quantity in adjustment becomes greater with enhancement of the compression efficiency, there is the possibility that a quantity to be adjusted may be equal to a quantity which cannot be neglected as compared to the magnitude of an input signal in the efficient compression technique.